As it passed through Parliament the EU Withdrawal Bill was amended and the final version of Section 13 of the Act is the provision which now makes clear that Government will not be able to ratify the withdrawal agreement unless four conditions have been met:

the negotiated withdrawal agreement (the divorce agreement), the agreement in principle on the substance of the framework for the future relationship between the EU and the United Kingdom after withdrawal, and a statement that political agreement has been reached, mu st all have been published and laid in Parliament

the negotiated withdrawal agreement and the framework for the future relationship have been approved by a resolution of the House of Commons on a motion moved by a Minister of the Crown

a subsequent debate has taken place in the House of Lords

Parliament has passed legislation to implement the withdrawal agreement.

The legislation raises two interesting questions; what will/should the resolution say and what happens if the four conditions are not met and the Government cannot ratify the agreement.

The resolution required by 13(1)(b), under the Standing Orders of the House of Commons, would be expected to be a "substantive motion" which is a motion that calls for action (or expresses an opinion). The best guess is that these debates may not take place until the week before Christmas. Substantive motions can usually be amended, by MPs tabling an amendment and then moving the amendment in the Chamber. The MPs Guide to Parliament suggests that the wording for an amendment might be: “to leave out (the words you want to omit from the motion) and insert (your words)”, or, if you want to leave out more than six words, “from ‘x’ to ‘y’ and insert (your words).” The amendment must: be relevant to the original motion; not achieve the same effect as voting against the motion; not be unintelligible, ungrammatical, vague, or mocking"

It seems highly likely that MPs will want to table amendments against the substantive motion and, depending on how votes are cast, the resulting final motion which is "passed" may be problematic for Government. If the Government does not get the motion passed in the way it wants it may have technically fulfilled the statutory requirement but in a way which hampers the ratification. For this reason Ministers are insisting that no amendments will be allowed to be made, (see Dominic Raab's letter of 10 October to the Chair of the House of Commons Procedure Committee)

Sir David Natzler (Clerk of the House of Commons) has suggested that if this approach is taken then "stand-alone consecutive motions" could be debated instead, he refers to the consecutive motion idea being used twice before to present a series of options to MPs relating to reform of the House of Lords in 2004 and 2007. He gave evidence to the Brexit Select Committee on 31 October 2018.

Given the level of disagreement anticipated for these debates it is quite possible that the resulting motion or ratifying legislation could be subject to urgent legal challenge. This could arise if the Government believe that that it has a mandate that allows them to ratify and others do not, particularly if the issue of amendment has been restricted. The Institute for Government has observed "If the motion were amended and someone did believe that this stopped the Government from ratifying under the terms of Section 13, then they could challenge the Government’s decision to ratify the withdrawal agreement before the High Court. (This would be the same for amendments to the legislation the Government brings forward to implement the withdrawal agreement, after the vote on the motion.)".

Of course if the Government does not receive the straightforward approval, or any approval of the deal that it will be seeking, then the Brexit Agreements cannot be ratified and the prospect of a no deal Brexit will become a very real probability. The legislation provides a timetable which requires the Government to make a statement to Parliament in the event that the motion is not approved. Parliament would have a further chance to vote on those plans but only on a motion to be expressed "in neutral terms". In the alternative, perhaps, another motion may have called for a second referendum (the so called "People's vote") or votes of no confidence may be triggered, leading to calls for a General Election. The priority then may revert to whether an extension to the two year Article 50 time limit can be agreed across the EU to buy the UK more time to implement the MPs votes. There are just 68 working days from 20 December to 29 March 2019.